That's not really true anymore, though.
Batted ball velocities, advanced metrics - it's far, far more accurate at predicting success then we were even 10 years ago. The result has been a much more direct way to success via farm systems. While we can cherry pick busts, the fact is that the Tampa Bay Rays, Giants, various versions of the Red Sox, Cardinals, Dodgers, Royals, Cubs, Indians... 100% the Astros - have almost been built exclusively from within and in consort with a few key outside investments. Even teams that got close and didn't quite get there (Blue Jays, Orioles, Mets) - were primarily home grown w/ measured investments here and there.
Acting like building from within is a lousy strategy or somehow inherently more risky is just not true. It's a lot less risky. If it wasn't true, everyone wouldn't be doing it. The Yankees have what - 9 home grown active roster position players on their roster? I mean jeez.